DESCRIPTION: The broad aim of this proposal is to examine the effect of trunk motion on the head and neck during stabilizing reactions. The control of head stability has been examined in relation to rotations at the axis of the head, where experimental disturbances have been simplified so that the trunk is fixed and the only compelling motion is angular. Such a focus may be irrelevant for most functional activities because the principle axis of rotation shifts with posture. Natural movement also incorporates combinations of angular and linear forces. Discrepancies between these experimental controls and functional movement may well underlie conflicting theories about the role of the vestibular system in the generation and maintenance of responses to instability, and can interfere with the clinician's ability to apply research findings for successful therapeutic interventions. In this proposal, strategies of head and trunk stabilization during rotations in the sagittal plane will be explored across several axes of rotation. If control of trunk movement is the primary goal of balance corrections, then the strategy employed to control head movement should be mechanically linked to that of the trunk rather than to stimulus properties or the attentional state of the subject. Seated, sagittal plane rotations will be performed in the dark when only the head and neck are free to move, and when the head, neck, and trunk are free to move. The axis of rotation will be placed either at the skull, the shoulder, or the hip to vary mechanical demands and linear acceleration signals. Tests of both healthy adults and patients with bilateral labyrinthine deficit will reveal vestibular effects and compensatory strategies. Different attentional states with predictable and random stimuli will reveal the effects of prediction. Neck and trunk muscle EMG activity, linear acceleration, angular velocity, and torques exerted on the support surface will be recorded to obtain information about the neural response properties. Frequency and time domain analyses will be employed to examine persistence in the magnitude and phase relations between the segments, which will provide insights into the limits of the neurophysiological or mechanical mechanisms. A biomechanical model will be developed to infer active biomechanics by subtracting out passive properties, and for comparison of predicted and empirical findings. The results of these studies will begin to resolve controversies concerning the relative importance of neural and mechanical controls, and will bring studies of postural control closer to natural destabilizing conditions for more direct therapeutic applications.